|  | 4.1 | LAN Design Goals and Components
| 4.1.5 | Why contention is an issue with Ethernet
You should decide carefully on the selection and placement of networking devices to be used in the LAN in order to decrease the collision detection and media contention on a network. Contention refers to excessive collisions on Ethernet caused by too many devices, each with a great demand for the network segment. The number of broadcasts becomes excessive when there are too many client packets looking for services, too many server packets announcing services, too many routing table updates, and too many other broadcasts dependent on the protocols, such as Address Resolution Protocol (ARP). An Ethernet node gets access to the wire by contending with other Ethernet nodes for the right to do so. When your network grows to include more nodes on the shared segment or wire, and these nodes have more and more messages to transmit, the chance that a node will contend successfully for its share of the wire gets much worse, and the network bogs down. The fact that contention media access does not scale or allow for growth, is Ethernet's main disadvantage. As shown in the Figure, as traffic increases on the shared media, the rate of collisions also increases. Although collisions are normal events in Ethernet, an excessive number of collisions will (sometimes dramatically) reduce available bandwidth. In most cases, the actual available bandwidth is reduced to a fraction (about 35% to 40%) of the full 10 Mbps. This reduction in bandwidth can be remedied by segmenting the network by using bridges, switches, or routers.
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